diff --git a/packages/prompts-core/src/index.ts b/packages/prompts-core/src/index.ts
index af788116b..639fa86a7 100644
--- a/packages/prompts-core/src/index.ts
+++ b/packages/prompts-core/src/index.ts
@@ -8,4 +8,4 @@ export type {
 } from "./types"
 export { resolveVariant } from "./variant-resolver"
 export type { ResolveVariantInput } from "./variant-resolver"
-export { loadPrompt, PromptFileNotFoundError } from "./loader"
+export { loadPrompt, PromptFileNotFoundError, PromptPathTraversalError } from "./loader"
diff --git a/packages/prompts-core/src/loader.test.ts b/packages/prompts-core/src/loader.test.ts
index 4017e1969..2842ab640 100644
--- a/packages/prompts-core/src/loader.test.ts
+++ b/packages/prompts-core/src/loader.test.ts
@@ -1,7 +1,7 @@
 import { describe, expect, test } from "bun:test"
 import { dirname, join } from "node:path"
 import { fileURLToPath } from "node:url"
-import { loadPrompt, PromptFileNotFoundError } from "./loader"
+import { loadPrompt, PromptFileNotFoundError, PromptPathTraversalError } from "./loader"
 import type { PromptSource } from "./types"
 
 const fixtureSource: PromptSource = {
@@ -50,6 +50,22 @@ describe("loadPrompt", () => {
     expect(expectError(error).message).toContain("test-prompt/missing")
   })
 
+  test("#given prompt name escapes source directory #then rejects path traversal", async () => {
+    const error = await captureError(() =>
+      loadPrompt({ source: fixtureSource, name: "../test-prompt", variant: "default" })
+    )
+
+    expect(error).toBeInstanceOf(PromptPathTraversalError)
+  })
+
+  test("#given variant escapes source directory #then rejects path traversal", async () => {
+    const error = await captureError(() =>
+      loadPrompt({ source: fixtureSource, name: "test-prompt", variant: "../../outside" })
+    )
+
+    expect(error).toBeInstanceOf(PromptPathTraversalError)
+  })
+
   test("#given runtime injection #then replaces placeholder in body", async () => {
     const prompt = await loadPrompt({
       source: fixtureSource,
diff --git a/packages/prompts-core/src/loader.ts b/packages/prompts-core/src/loader.ts
index 6d9a04218..df01b4579 100644
--- a/packages/prompts-core/src/loader.ts
+++ b/packages/prompts-core/src/loader.ts
@@ -1,6 +1,6 @@
 import { parseFrontmatter } from "@oh-my-opencode/utils"
 import { readFile } from "node:fs/promises"
-import { join } from "node:path"
+import { isAbsolute, relative, resolve } from "node:path"
 import type { LoadedPrompt, LoadPromptInput, RuntimeInjection } from "./types"
 
 export class PromptFileNotFoundError extends Error {
@@ -16,10 +16,21 @@ export class PromptFileNotFoundError extends Error {
   }
 }
 
+export class PromptPathTraversalError extends Error {
+  readonly name = "PromptPathTraversalError"
+
+  constructor(
+    readonly promptName: string,
+    readonly variant: string
+  ) {
+    super(`Prompt path escapes source directory for ${promptName}/${variant}`)
+  }
+}
+
 export async function loadPrompt<TFrontmatter = Record<string, unknown>>(
   input: LoadPromptInput
 ): Promise<LoadedPrompt<TFrontmatter>> {
-  const filePath = join(input.source.baseDir, input.name, `${input.variant}.md`)
+  const filePath = resolvePromptFilePath(input.source.baseDir, input.name, input.variant)
   const content = await readPromptFile(input.name, input.variant, filePath)
   const parsed = parseFrontmatter<TFrontmatter>(content)
   const body = await applyRuntimeInjections(parsed.body, input.inject ?? [])
@@ -33,6 +44,16 @@ export async function loadPrompt<TFrontmatter = Record<string, unknown>>(
   }
 }
 
+function resolvePromptFilePath(baseDir: string, promptName: string, variant: string): string {
+  const resolvedBaseDir = resolve(baseDir)
+  const filePath = resolve(resolvedBaseDir, promptName, `${variant}.md`)
+  const relativePath = relative(resolvedBaseDir, filePath)
+  if (relativePath.startsWith("..") || isAbsolute(relativePath)) {
+    throw new PromptPathTraversalError(promptName, variant)
+  }
+  return filePath
+}
+
 async function readPromptFile(promptName: string, variant: string, filePath: string): Promise<string> {
   try {
     return await readFile(filePath, "utf8")
diff --git a/packages/prompts-core/src/variant-resolver.ts b/packages/prompts-core/src/variant-resolver.ts
index f2f5e1d81..605924c5f 100644
--- a/packages/prompts-core/src/variant-resolver.ts
+++ b/packages/prompts-core/src/variant-resolver.ts
@@ -8,15 +8,17 @@ import {
 } from "@oh-my-opencode/model-core"
 import type { VariantTable } from "./types"
 
+type ModelMatcher = (modelID: string) => boolean
+
 export type ResolveVariantInput = {
   readonly modelID?: string
   readonly agentName?: string
   readonly variants: VariantTable
 }
 
-const PLANNER_AGENT_NAMES = new Set(["prometheus"])
+const PLANNER_AGENT_NAMES: ReadonlySet<string> = new Set(["prometheus"] as const)
 
-const MODEL_MATCHERS: Record<string, (modelID: string) => boolean> = {
+const MODEL_MATCHERS: Readonly<Record<string, ModelMatcher>> = {
   gpt: isGptModel,
   gemini: isGeminiModel,
   kimi: isKimiK2Model,
@@ -43,10 +45,7 @@ export function resolveVariant(input: ResolveVariantInput): string {
 
   if (variantNames.includes("default")) return "default"
 
-  const firstVariant = variantNames[0]
-  if (firstVariant !== undefined) return firstVariant
-
-  throw new TypeError("resolveVariant requires at least one prompt variant")
+  return variantNames[0]
 }
 
 function isPlannerAgent(agentName: string | undefined): boolean {